▎ 摘　　要

When electrons are confined in two-dimensional materials, quantum-mechanically enhanced transport phenomena such as the quantum Hall effect can be observed. Graphene, consisting of an isolated single atomic layer of graphite, is an ideal realization of such a two-dimensional system. However, its behaviour is expected to differ markedly from the well-studied case of quantum wells in conventional semiconductor interfaces. This difference arises from the unique electronic properties of graphene, which exhibits electron - hole degeneracy and vanishing carrier mass near the point of charge neutrality(1,2). Indeed, a distinctive half-integer quantum Hall effect has been predicted(3-5) theoretically, as has the existence of a non-zero Berry's phase ( a geometric quantum phase) of the electron wavefunction - a consequence of the exceptional topology of the graphene band structure(6,7). Recent advances in micromechanical extraction and fabrication techniques for graphite structures(8-12) now permit such exotic two-dimensional electron systems to be probed experimentally. Here we report an experimental investigation of magneto-transport in a high-mobility single layer of graphene. Adjusting the chemical potential with the use of the electric field effect, we observe an unusual half-integer quantum Hall effect for both electron and hole carriers in graphene. The relevance of Berry's phase to these experiments is confirmed by magneto-oscillations. In addition to their purely scientific interest, these unusual quantum transport phenomena may lead to new applications in carbon-based electronic and magneto-electronic devices.